This invention relates to the manufacture of chains and in particular is concerned with a method and apparatus for heating wire pins by means of electric resistance heating with electrodes that can be positioned on the radial face of the wire pin.
It is generally known to heat wire pins -- which are subsequently bent in a warm condition to form chain links -- with the aid of the electric resistance heating. In such an operation particular care must be taken regarding the connection between the wire pins and the electrodes since the contact established at the location of connection has great importance regarding the economical nature of the heating process, on the one hand, and the quality of the chain links formed from the heated wire pins, on the other hand. From the electric point of view, it is best to position the electrodes radially on the side of the pin ends. In this manner relatively large electrodes may be applied which make possible a high current transfer and thus result in short heating periods. This arrangement, however, has the disadvantage that the pin sides which receive the electrodes must have a clean and uniform external contour. This requirement makes it necessary that, for example, rolled pins be submitted to subsequent treatment in order to ensure the necessary clean surface. An accuracy of shape can subsequently not be corrected so that imperfect contacts between electrode and pin face result in burns which adversely affect the quality of the chain link subsequently made from the wire pin.
According to another known method, the electrodes are positioned on the end faces of the pins. Since the pins are generally made by means of a sawing or cutting operation, the contact faces are metallically always clean. If, because of surface irregularities, burns occur, the latter are removed during the welding of the bent chain link, so that such burns will not adversely affect the quality of the chain links. This method, however, is disadvantageous with respect to the previously outlined method in that the maximum area of engagement for the electrodes can be no greater than the cross-sectional area of the pin. Since, dependent upon the severing operation with which the individual pins are made from the continuous wire, the end face of the pin ends represents, a contact face which is smaller than the cross section of the pin (this is the case, for example, when the pin is separated from the continuous wire with simultaneous rounding off the pin end), necessarily longer heating periods result. The advantages of this method regarding the quality of the chain links are, however, very significant particularly in the manufacture of heavy-duty, high-quality chains.